There is an ever increasing need for high purity gases and liquids for use in various chemical, medical and pharmaceutical, and manufacturing processes, specifically in microelectronics, e.g., lithography, epitaxy, and thin film processing. One of the challenges facing today's high purity gas and liquid users is measuring contaminants in their process gas. Contaminants present in small amounts, such as parts-per-trillion (ppt) concentrations, can adversely affect the processes, especially over long periods of chronic exposure. Contaminants may occur in impulse or random events which equipment monitoring response time or random nature prevents measurement and containment. By providing purification on line, impulse events may be smoothed and low to undetectable contaminant levels are concentrated in a purifier.
At the ppt and sub-ppt levels of contamination, continuous process monitoring is not feasible with current analytical methods and technologies. Direct injection methods may be imprecise, inaccurate, or cost prohibitive. An example of direct injection method capable of continuous contaminant monitoring is found in U.S. Pat. No. 6,547,861, which describes an APIMS detection method capable of reaching 25 ppt. See, U.S. Pat. Nos. 6,550,308, 6,397,660, 6,418,781 and 5,304,796. However, this system is large, expensive, and requires a skilled analytical chemist or technician to operate. Concentration methods are often employed to monitor contaminants over longer time periods. This method involves placing a concentration device, such as a thermal desorption tube (TDT), into contact with the process fluid. After a certain time period the TDT is removed from the process and sent to an analytical lab for testing. While the TDT method provides low cost analysis of chronic contamination in a process, it suffers from many drawbacks. TDTs are normally specific to a certain class of contaminants, usually hydrocarbons, and often suffer from low contaminant capacities, reducing sampling times and requiring frequent replacement. TDTs do not generally meet the efficiency and capacity requirements of purifiers. TDTs are often incompatible with ultra-high purity (UHP) fluid delivery systems. TDTs are not required elements of fluid delivery systems and, thus, increase the cost and complexity of any fluid delivery system. When installed parallel to the process fluid delivery line, TDTs bleed gas from the process, thus increasing gas use and requiring adequate venting. TDTs are easily cross-contaminated during installation and removal from process fluid lines. The trapping efficiencies of many TDTs are not very high, resulting in lower detection limits of 100 ppt.
There are many possible sources of contamination during industrial processes. Source gases and liquids may contain contaminants. System leaks can generate environmental contaminants. Off-gassing and permeability of tool materials, mainly plastics but also ceramics and stainless steel can be a source of system contamination. Contaminants may arise from the by-products of reactions in the delivery path or UV light induced reactions, e.g., in photolithography. Such potential contaminant sources and their effects on the manufacturing process provide impetus for using point-of-use (POU) purifiers.
Examples of POU purifiers include inorganic adsorbents, including zeolites, silica, alumina and transition metal-based adsorbents; palladium cells; organic polymers, which can be imbedded with adsorbent materials; and others known to those skilled in the art. Exemplary POU purifiers are those taught in U.S. Pat. Nos. 6,391,090; 6,361,696; 6,241,955; and 6,059,859. The POU purifier selected depends on the source of the gas or liquid to be passed through the purifier and the sources of contaminants typically present in the process. Specific contamination issues may vary broadly between gases and liquids, as well as between different classes of gases or liquids. For example, light hydrocarbons (LHC) generally come from impure source gases. Nitrogenous contamination, moisture and organic solvent vapors are generally present in cleanrooms, but may be condensed on surfaces or dissolved in source liquids. Heavy hydrocarbons and refractory compounds (e.g., siloxanes) generally come from plastics, lubricants, and seals used in tools. Oxygen (O2) and carbon dioxide (CO2) typically come from the environment, i.e. leaks in the fluid delivery system. As different contaminants tend to arise from different points in a process, examining contaminants adsorbed by purifiers indicates sources of contamination in a process. The information generated by such an analysis can be used to improve process control. Identification of contamination sources allows for the isolation of the step to determine the effects on the process.
While many purifiers remove contaminants, in most cases the contaminant molecules are destroyed or otherwise modified as they are removed. Therein the contaminant molecules cannot be released from the purifier material or are not released in a discernible form or reproducible concentration. If all hydrocarbon contaminants are converted into carbon dioxide and water or if all sulfur-containing contaminants are released as SO2 or H2S, the true nature of the contaminant as it was adsorbed by the purifier material is indiscernible. For example, non-evaporable getters are thought to adsorb nitrogen to form metal nitrides, oxygen to form metal oxides, hydrogen to form metal hydrides, and hydrocarbons to form metal carbides, each of which can migrate from the surface into the bulk of the getter alloy. In this case the contaminant species cannot be desorbed from the purifier in a chemical state identical to that found in the gas or liquid stream. Furthermore, probably as a result of the solid state diffusion process that occurs in these and many other purifier materials, desorption of the contaminants from the purifier material, even as a chemical relative of the original contaminant species, requires high temperatures, is inaccurate, and involves hazardous conditions.